In semiconductor memory devices, there is an EPROM (Erasable Programmable Read Only Memory) which can not only electrically write data but also erase the data by irradiating an ultraviolet ray on a transistor holding the data. Ordinarily, a semiconductor has memory cells which each store 1-bit data are arrayed in a matrix and can perform a data writing operation and a data reading operation for a memory cell in a designated position, for example, by designating a row and a column by using an address decoder. For example, a word line of each memory cell is connected to a Y address decoder and, then, the Y address decoder applies a row selection signal to the word line corresponding to an inputted Y address. On the other hand, a bit line of each memory cell is connected to an X address decoder and, then, the X decoder inputs data from outside to the bit line corresponding to an inputted X address or transmits the data from the bit line to an output circuit.
In the EPROM, a threshold voltage Vt of an FET is changed by injecting charges into a floating gate electrode or the like provided on a channel of an MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) via a gate oxide film. Since once-injected charges are held for a long period of time, data can be stored by a difference of the Vt. On the other hand, the charges can be discharged by irradiating the ultraviolet ray on the floating gate electrode and, by such discharging, the data can be erased.
A conventional EPROM is provided with one piece of such MOSFET as described above per memory cell. By applying a selection signal to a gate electrode (control gate electrode) of the MOSFET via a word line, the data written in the memory cell is read out on a bit line as a current in accordance with the Vt. An output circuit compares the thus-read out current with a given reference value and judges whether the data is “0” or “1”.
On this occasion, injection of the charges into the floating gate electrode is performed by applying a high voltage between the control gate electrode connected to the word line and a drain diffusion layer connected to the bit line and, then, allowing the charges flowing out of the drain to have a high energy. Therefore, same charges as majority carriers of the drain are injected into the floating gate electrode. As a result, the thus-injected charges have an effect to shield a potential fluctuation which the control gate electrode puts on a channel and, then, the Vt of the MOSFET in a charge injected state becomes higher than that in a state of being not injected with the charges and, therefore, a data signal current read out into the output circuit becomes relatively small in quantity.
In a data detecting method in which, as described above, the data signal current read out from the memory cell via the bit line is compared with the predetermined reference value in the output circuit, it was necessary to provide a comparatively large margin for variance, fluctuation or the like among data signal currents. In contrast, considered is a constitution in which 2 pieces of the above-described MOSFETs capable of writing data are provided per memory cell and, then, a Vt of one of the MOSFETs which is selected in accordance with data to be written in the memory cell is allowed to be changed by injecting charges. In this constitution, for example, a Tr 1 and a Tr 2 are provided as 2 MOSFETs and, then, the charges are injected only into the Tr 1 against a data value “1” while only into the Tr 2 against a data value “0”. By such operations as described above, the Vt of one of the MOSFETs is set higher while that of the other lower and, then, a magnitude relation between data signal currents flowing in respective MOSFETs is shifted in accordance with data values. When a difference of the magnitude relation can be detected by the differential amplifier, there is a merit in that the margin against the above-described variation, fluctuation or the like among individual data signal currents can be made small.
However, in a state in which data is erased by ultraviolet ray irradiation, that is, in an initialized state, since Vts of two MOSFETs are same with each other, a given value can not be obtained in an output of the differential amplifier. For this account, there is caused an inconvenience in that it is difficult to confirm whether the data is erased or not (or whether used or not used). Then, when the MOSFET constituting the memory cell is such a type capable of electrically erasing or rewriting data as is used in a flash memory, it is possible to previously write an initializing data at a manufacturer's side before shipment such that the output of the differential amplifier does not become unstable. However, in order to erase the data in the EPROM, it is necessary to irradiate the ultraviolet ray thereon and, once the initializing data is written, it is difficult that a user rewrites the initial data and, then, stores a desired data. Therefore, the above-described inconvenience is an intrinsic problem in the ultraviolet erasable semiconductor memory device.